A Hewlett-Packard Transition Analyzer was borrowed on consignment as part of an engineering development demonstration. The Mims cavity was analyzed using this instrument as a passive filter. Its S-parameters were measured subject to varying design parameters. The results confirmed that the device functions as a bandpass filter using a fractional wavelength resonator spanned by two quarter wavelength shunts. Proof of its coupling mode (electric field component of TE mode) was established and salient features for design modification and optimization of bandwidth were identified.